The structure of oriented multilayers of the zwitterionic phospholipid palmitoyloleoylphosphatidylcholine containing nonionic surfactants (oligo(oxyethylene glycol) monododecyl ethers, C(12)E(n) with n = 2, 4, and 6) was studied by X-ray and neutron diffraction. Investigations were done at T = 20 degrees C in the lamellar liquid-crystalline phase with surfactant/lipid molar ratios R(A/L) = 0.5 and 1 and hydration levels corresponding to 85 and 97% relative humidity. The transbilayer distribution of the surfactant was determined by neutron diffraction using selectively deuterated C(12)E(n). The incorporation of surfactant causes a decrease of the repeat distance and of the thickness of the hydrocarbon core of the membrane. The effect increases with the number of oxyethylene units and with the concentration of the surfactant as well as with the relative humidity. The alpha-methylene group of the surfactants is anchored near the boundary of the hydrophobic core. The oxyethylene moieties are mainly located in the polar membrane/water interface region. The impact of molecular disorder on the diffraction data is discussed.